Remotely controlled multirotor aircraft controlled by human voice

ABSTRACT

A multi-rotor remote control aircraft for capturing audio and/or video signals and a method for remote controlling the aircraft by way of voice commands. The aircraft and method mitigate the effects of audio noise produced by motors and propellers on reception and detection of the voice commands. Audio acquisition components are provided for receiving the voice commands while noise acquisition components are devoted for capturing the environmental noise. The mitigation of the noise effects is achieved by filtering and a cancellation technique. With the cancellation technique, the noise part contained in the signal captured by the noise acquisition components is equalized to the noise part contained in an audio signal carrying the voice commands and then it is subtracted from the audio signal.

FIELD OF THE INVENTION

The present invention relates to a multi-rotor remote control aircraftfor capturing audio and/or video signals and a method for remotecontrolling said aircraft.

BACKGROUND

A multi-rotor aircraft (like a bicopter, a tricopter, a quadcopter, ahexacopter, an octocopter or the like) is inherently unstable, so itrequires a constant engine speed adjustment to maintain the orientationset by the pilot and/or by the flight control system.

Speed adjustment is usually carried out by special regulators (such asProportional, Integral and Derivative regulators—in short PIDs) actingseparately on each of the three axes of rotation (pitch, roll and yaw)of the aircraft, so as to maintain angular rotational speeds along theseaxes as near as possible to the values selected by the pilot through aremote control device such as a remote control, a radio control or thelike, and/or by the flight control system.

The awkward operation of this remote control device, typical ofinexperienced pilots, inevitably causes security problems and/oroscillations of the aircraft, which make it difficult to control theaircraft and to take pictures, since under certain exposure conditionsthe photos and the videos captured by the video capture media areaffected by wobble, also known as ‘Jello’ effect, when sensors areequipped with Rolling Shutters.

Therefore, for this kind of aircrafts, there is a need to have a remotecontrol, easy to use and allowing inexperienced users to avoid abruptmovements of the aircraft. Regular radio control devices are in factcomplex to use and require some training. Moreover, they have to becarried together with the aircraft and are a burden for the user.

SUMMARY OF THE INVENTION

The present invention aims to solve these and other problems byproviding a voice control method whereby the user can control theaircraft by voice commands using words of common language, such as“forward”, “back”, “right”, “left”, “stop” or “turn to the left”,“rotate to the right”, “slide to the left”, “slide to the right” or thelike. Accordingly, the aircraft comprises audio acquisition meansadapted to receive an audio signal carrying the user's voice, and speechconversion means for converting voice commands into flight controlsignals. This audio acquisition means can be preferably set to recognizespeech in different languages, one of them being selected by theaircraft's user.

However, the aircraft motors and their propellers produce audio noisethat may affect the entire voice band. In fact, the harmonics of thepropeller/motor shaft rate and the blade passing frequency have veryhigh amplitudes up to the fifth harmonic of the shaft rate (seeExperimental Study of Quadcopter Acoustics and Performance at StaticThrust Conditions, by W. Nathan Alexander et al., AeroacousticsConferences, 30 May-1 Jun. 2016, Lyon, France, 22nd AIAA/CEASAeroacoustics Conference). Therefore, the fifth harmonic of the shaftrate of 18,000 RPM (300 Hz) has a frequency of 1,500 Hz, which is in thecore of the band 300 Hz to 3400 Hz used for instance in communicationfor the telephony voice service (see https://en.wikipedia.org/wiki/Voicefrequency).

In order to reduce the energy of the unwanted noise components affectingthe voice signal, the invention teaches two complementary techniques:filtering out from the voice signal the noise components by means ofstopband filters, and cancelling the noise components by means of acancellation technique. Preferably, the filtering technique is used forattenuating the unwanted components that are not in the core of thevoice band, while the cancellation technique can be used also forfrequencies that are inside the voice band.

Both techniques exploit the presence, in the environmental noise, oftones related to the shaft rate of the aircraft motors. They allow theidentification of the frequencies of noise components to be removed andthe assessment of the general characteristics of the noise.

The shaft rate of the aircraft motors may be derived from the motorscontrol signals or from a spectral analysis of the audio signal. For abetter and easier identification of the environmental noise components,dedicated acquisition means may be used to pick up a noise signal with aminimum component of the user's voice signal.

In the case of the cancellation technique, narrow passband filters maybe used with their passband centered around the frequencies of unwantedcomponents (shaft rates, harmonics of the shaft rates, and others), forextracting corresponding signal components from the noise signal and theaudio signal. Each signal component extracted from the noise signal isthen compared against the corresponding one extracted from the audiosignal, and data of relative amplitude, phase, and delay are computed.With those data, a transfer function is defined for a filter equalizingand aligning the noise part contained in the noise signal to the noisepart contained in the audio signal. Then the noise signal, processed bysaid transfer function, is subtracted from the audio signal.

In this way, on board the aircraft it is possible to acquire audiosignals carrying voice commands and translate the voice commands intocontrol signals for the flight control means.

Further advantageous features of the present invention are set forth inthe attached claims.

BRIEF DESCRIPTION OF THE DRAWINGS

These features and further advantages of the present invention willbecome more apparent from the description of an embodiment thereof,shown in the accompanying drawings, provided purely by way ofnon-limiting example, wherein:

FIG. 1 shows a perspective view of a multi-rotor aircraft equipped withsound acquisition means;

FIG. 2 shows block diagram of noise reduction means comprised in avoice-controlled aircraft according to the invention;

FIG. 3 shows an illustrative shape of the amplitude characteristic of apassband filter with a narrow stopband;

FIG. 4 shows an illustrative shape of the amplitude characteristic of amulti-passband filter with equally spaced passbands and constantfractional bandwidth.

DETAILED DESCRIPTION OF THE INVENTION

In this description, any reference to “an embodiment” will indicate thata particular configuration, structure or feature described in regard tothe implementation of the invention is comprised in at least oneembodiment. Therefore, the phrase “in an embodiment” and other similarphrases, which may be present in different parts of this description,will not necessarily be all related to the same embodiment. Furthermore,any particular configuration, structure or feature may be combined inone or more embodiments in any way deemed appropriate. The referencesbelow are therefore used only for simplicity sake, and do not limit theprotection scope or extension of the various embodiments.

FIG. 1 shows a perspective view of a multi-rotor aircraft 100 equippedwith two kind of sound capturing means: audio acquisition means 101adapted to receive an audio signal carrying a control command for theaircraft 100, and noise acquisition means 102 for receiving a noisesignal coming from the environment surrounding the aircraft 100.

The audio acquisition means 101 may be a microphone or a set ofmicrophones designed for capturing the voice of the aircraft user. Theymay have directional properties for picking up sound predominantly fromone direction, in particular from the direction where the aircraft useris located. Preferably, the audio acquisition means 101 may beimplemented by an array of sensing elements in conjunction withbeamforming algorithms, whose pointing direction can be controlled by acontrol signal. Moreover, the aircraft 100 may be equipped with voicesource localization means, configured for calculating the direction fromwhich the audio signal, carrying the user's voice, comes.

The voice source localization means may operate by processing thesignals captured by the sensing elements of the audio acquisition means101. They, however, may also utilize other sensing elements, possibly incombination with those of the audio acquisition means 101. With thecomputed direction from which the user's voice comes, the voice sourcelocalization means issue a control signal for the acquisition means(101) to point their beam in said calculated direction.

MEMS (MicroElectro-Mechanical Systems) sensing elements may be used toimplement small, high-performance microphones with high dynamic range.However, the skilled person may suggest other technologies and otherstructure, for implementing the audio acquisition means 101 and thevoice localization means without departing from the teaching of thepresent invention.

The noise acquisition means 102 may be a microphone or a set ofmicrophones and/or vibration sensors, and are configured for capturingthe environmental noise.

Both the audio acquisition means 101 and the noise acquisition means 102capture the voice of the aircraft user and the environmental noise, butwith different relative levels and characteristics. Analyzing thesedifferences, it is possible to distinguish the voice components from thenoise components and subtract the noise components from the audio signalso as to obtain an audio signal sufficiently clean for reliablydetecting simple voice commands.

The assembly 200 for performing these functions is now described alsowith reference to FIG. 1 and FIG. 2, considering that the multi-rotoraircraft 100 is preferably equipped with

-   -   at least a first motor (preferably four), which can be        controlled by a motor control signal and coupled to a first        propeller, capable of generating a thrust for making the        aircraft 100 flying,    -   flight controls means 213, such as a PixHawk® model unit        produced by 3D Robotics, adapted to receive a command signal        defining an attitude and/or movement and/or direction of the        aircraft, and issue at least one control signal for controlling        the flight of the aircraft, by means of said at least first        motor, on the basis of said control signal.    -   the audio acquisition means 101, such as a microphone coupled to        an analog-to-digital conversion unit (ADC), capable of acquiring        an audio signal carrying an audio command for the aircraft 100;    -   the noise acquisition means 102 apt to receive a noise signal        coming from the environment surrounding the aircraft 100;    -   an input filter 203 for, such as a high pass filter and/or        bandpass filter, a notch filter, a Finite Impulse Response (FIR)        or Infinite Impulse Response (IIR) filter or other, for        filtering the audio signal carrying an audio command and in        particular for reducing the energy of a noise component in a        first frequency band;    -   a delay filter 204 for possibly delaying the filtered audio        signal coming out from the input filter 203 on the basis of a        delay datum set by the noise equalization processor 210;    -   cancellation means 205 adapter to subtract from the filtered        audio signal a noise component so as to cancel the noise        component from the audio signal;    -   speech conversion means 206 apt to convert an audio signal        carrying an audio command into a command signal for the flight        control means 213;    -   filter setting means 207 adapter to set parameters defining        stopband or passband filters, comprising multiband filters, on        the basis of reference frequency data received at its input from        a noise component selector 211;    -   audio component filter 208, implementing a passband filter or a        multi-bandpass filter specified by filter setting means 207, for        extracting signal components from the audio signal;    -   noise component filter 209, implementing a passband filter or a        multi-bandpass filter specified by filter setting means 207, for        extracting signal components from the noise signal;    -   noise equalization processor 210, such as a CPU or a set of        CPUs, preferably operating in a programmable manner and        executing specific instructions, configured for setting data        defining a delay and a noise equalization transfer function of a        filter equalizing the noise part of a signal component extracted        from a noise signal to the noise part of a signal component        extracted from an audio signal, on the basis of said extracted        components and/or said noise signal and/or said audio signal;    -   noise component selector 211 configured for setting at least one        reference frequency of a noise component of the noise signal, on        the basis of at least one motor control signal received from the        flight control means 213, representing the rotation rate of said        first motor, and/or on the basis of an analysis of the noise        signal;    -   equalization filter (212) configured for implementing the noise        equalization filter transfer function set by the noise        equalization processor (210).

The input filter 203 may comprise an analog filter, i.e. a filter madeof discrete electronic components (such as, for example, resistors,capacitors, and inductors, preferably of a variable type), and/or adigital filter, i.e. a set of instructions implementing a filteringalgorithm. Its transfer function may be a combination of a passbandtransfer function, encompassing the voice band, with a stopband transferfunction or with a multi-stopband transfer function, for attenuating onenarrowband noise component or a number of narrowband noise components.FIG. 3 shows an example amplitude characteristic of such a transferfunction relevant to the single stopband case.

In a first embodiment of the invention the attenuation of the noiseinterference is achieved only by filtering the audio signal by means ofthe input filter 203 (FIG. 2). In the case where the motors are runningat the same speed, this is achieved by the operation of:

-   -   audio acquisition means 101 apt to receive an audio signal        carrying voice commands for said aircraft 100 and at least one        unwanted noise component;    -   noise component selector 211 configured for deriving the        frequency of said at least one noise component of the noise        signal on the basis of the motor control signals received from        the flight control means 213, which represents the rotation rate        of the motors; it is to be noted that, preferably, the frequency        of said noise component is the frequency of the harmonic of the        motor rotation rate closest to the lower edge of the voice        frequency band and, preferably, outside the voice frequency        band; therefore the noise component selector 211 preferably        derives the motor rotation rate from the motor control signal        received from the flight control means 213 and computes the        frequency of the noise component to be attenuated according to        the above criteria; however, the narrow stopband characteristic        may also have its stopband in the voice band;    -   filters setting means 207 configured for setting data defining        at least one stopband characteristic of an input filter 203,        wherein the stopband of said at least one stopband        characteristic includes said selected frequency or an harmonic        thereof; assuming, for instance, that the input filter 203 is        implemented by a fixed passband filter cascaded with a notch        filter, the filters setting means 207 simply set the filter        notch frequency; however, the skilled person knows other ways to        implement the input filter 203;    -   input filter 103 configured for implementing said at least one        stopband characteristic and filtering said audio signal; it is        to be noted that the stopband characteristics may be implemented        in combination with other filtering characteristics (e.g.        high-pass, band-pass, or others);    -   speech conversion means 206 apt to convert an audio signal,        carrying voice commands, into a command signal for said flight        control means 213.

The speech conversion means 206 may be configured to generate thecontrol signal on the basis of said filtered audio signal by executing aset of instructions implementing a speech recognition algorithm, such asan algorithm reproducing the operation of a Neural network appropriatelytrained, preferably of the Deep Neural Network (DNN) type. In fact,using a neural network, it is possible to overcome the lack of audiosignal entailed by the filtering needed to remove the noise generated bymotors and propellers, even if the filtering characteristics varyaccording to the motors rotational speed.

This allows controlling the aircraft 100 using only the voice, withoutusing a remote control as in the state of the art. In this way, thesecurity of the aircraft 100 is advantageously improved and theoscillations/vibrations of the aircraft caused by inexperienced usersare advantageously reduced, so as to render the quality of the imagescaptured by the video capture means independent of the user's pilotingskills.

More in detail, the recognition algorithm reads the filtered audiosignal that represents a voice command (such as “forward”, “back”,“right”, “left”, “stop” or “turn to the left”, “rotates to the right”,“slides to the left”, “slides to the right” or the like) imparted by theuser of said aircraft and outputs a control signal that, as describedabove, defines an attitude and/or movement and/or an aircraftorientation, such as an inclination of the aircraft along its axisand/or a movement along a particular direction and/or an orientationtowards a particular direction (e.g. expressed in degrees measuredclockwise from North). It should be noted that such a voice command ispreferably encoded in a digital audio encoding format (such as WAV, MP3or other) so it can be processed by digital processing media.

When the aircraft engines do not operate at the same speed, the noisecomponent selector 211 may derive a noise component frequency for eachof them or take an average in the case of close frequency values.

Accordingly, the filters setting means 207 may set data for the relevantfilter characteristics and the input filter 103 will implement therelevant stopbands.

A variant of the above described embodiment takes into account that therelationship between the motor control signal received from the flightcontrol means 213 and the actual rotation rate of the controlled motoris somewhat loose. According to this variant, the selection of thefrequency of the noise component to filter out further comprises theacquisition of the noise signal coming from the environment surroundingthe aircraft 100 by the noise acquisition means 102, while said noisecomponent selector 211 is configured for setting the frequency of saidnoise component on the basis of the motor control signal and/or on thebasis of at least one characteristic of the noise signal that said noisecomponent selector 211 detects. Said characteristic of the noise signalmay be a comb of tones, an expected sequence of tones, or somethingelse. In particular, it may be a high-level, narrowband component ofsaid noise signal, e.g. the highest narrowband component or asubharmonic thereof. Therefore, the frequency of the noise component tofilter out may be selected according to a coarse indication given by themotor control signal and a refinement derived from a characteristic ofthe noise signal; alternatively it may derived from a characteristic ofthe noise signal only.

A second embodiment of the invention comprises the features of the firstembodiment with its variants, as above described, with the addition of anoise cancellation technique, which consists in producing a noise signalequalized to the noise component contained in an audio signal andsubtracting such equalized noise component from the audio signal. Thiscancellation technique is achieved by the operation of

-   -   filters setting means 207 configured for additionally setting        data defining a passband characteristic that, in its passband,        includes the frequency of the selected noise component; the        additional data are set on the basis of a frequency (selected by        the noise component selector 211) of at least one noise        component and/or its harmonics;    -   noise component filter 209 and audio component filter 208        configured for implementing said passband filter and extracting,        respectively, at least one component from the noise signal and a        corresponding component from the audio signal coming from the        input filter 203;    -   noise equalization processor 210 configured for setting data        defining a delay and a noise equalization transfer function of a        filter equalizing the noise part contained in said at least one        component extracted from said noise signal to the noise part        contained in said corresponding component extracted from said        audio signal coming from the input filter 203, on the basis of        the signal components extracted by the noise component filter        209 and the audio component filter 208;    -   delay filter 204 configured for implementing said delay, which        is set by said noise equalization processor (210); this delay        should compensate for the delay that the noise component        extracted from the noise signal undergoes, through the        equalization filter (212), minus the delay that the audio filter        203 introduces on the audio path; in the case where this        difference gives a negative value, the noise equalization        processor 210 sets this delay to zero and adds a corresponding        delay to the noise equalization transfer function;    -   equalization filter 212 configured for implementing the filter        transfer function defined by the noise equalization processor        210;    -   cancellation means 205 configured for reducing, in the audio        signal delayed by said delay filter 203, the energy of the        unwanted noise component, on the basis of the output of the        noise equalization filter 212.

With this second embodiment of the invention, the reduction of the noisein the audio signal greatly improves. In fact, the noise equalizationprocessor 210 may define a number of points of the transfer functionequalizing the noise component of the noise signal to the noisecomponent of the audio signal and interpolate those points to providethe equalization over the entire voice band or a large part thereof.

In a variant of the second embodiment of the invention the passbandfilter characteristic may be a multi-passband with the passbands equallyspaced to cope with the harmonics of a fundamental frequency. Moreover,the passbands of the multi-passband characteristic may have the samefractional bandwidth (the ratio of the width of a frequency band to theband center frequency) to accommodate the possible frequency shift andjitter that may affect signal components proportionally to their ordinalnumber. This case is qualitatively illustrated in FIG. 4.

The same applies to the stopband filters, mainly dealt with in the firstembodiment of the invention: multi-stopband filter may be defined withstopbands equally spaced and/or having a same fractional bandwidth.

In another variant of the second embodiment, the data defining a delayand a noise equalization transfer function are set, by the noiseequalization processor 210, on the basis of the signal componentsextracted by said noise component filter 209 and said audio componentfilter 208 and/or on the basis of said noise signal and/or said audiosignal coming from the input filter 203. By comparing and analyzing bothsignal at various frequencies, the noise equalization processor 210 maybetter define the equalization transfer function over the entire voiceband or a large part thereof.

On the basis of the clean audio signal obtained at the output of thecancellation means 205, as explained above the speech conversion means206 generate a control signal for the flight controls means 213performing the steps of

-   -   executing a set of instructions implementing a speech        recognition algorithm, by converting the audio signal into a bit        string, and    -   generating the control signal based on said bit string.        What is described above, with reference to the annexed figure,        defines not only a multi-rotor aircraft 100 controlled by voice        command, but also an audio capture device capable of capturing        an audio signal carrying voice commands and transmitting said        commands, converted into suitable control signals, to a        controlled apparatus.

When the aircraft 100 is in an operating condition, the assembly ofelements shown in FIG. 2 executes a method for remote control of theaircraft 100 comprising the following phases:

-   a. a frequency selection phase, wherein, by means of a noise    component selector 211, the frequency of at least one noise    component of a noise signal is selected on the basis of the control    signal of a first motor, which represents the rotational speed of    said first motor that is comprised in the aircraft 100;-   b. a filters setting phase, wherein a first frequency band is set by    means of filters setting means 207 on the basis of said selected    frequency or an harmonic thereof;-   c. a filtering phase, wherein an audio signal acquired by audio    acquisition means 101 is filtered by an input filter 203 featuring a    stopband filter characteristic according to said first frequency    band, so as to reduce the energy of at least one component of said    audio signal having a frequency contained in said first frequency    band and generate a filtered audio signal;-   d. a command generation phase, wherein a command signal is    generated, by means of speech conversion means 206, on the basis of    said filtered audio signal, wherein said command signal defines an    attitude and/or a movement and/or an orientation of the aircraft    100;-   e. a control transmission phase, wherein said command signal is    transmitted to flight control means 213 for controlling the flight    of said aircraft 100.

In combination with the above-described features, the assembly ofelements shown in FIG. 2 may execute the following steps:

-   -   acquiring, by means of audio acquisition means 102, a noise        signal representing the environmental noise surrounding said        aircraft 100;    -   setting the frequency of at least one component of said noise        signal, on the basis of said motor control signal and/or at        least one characteristic of said noise signal that said noise        component selector 211 detects in said noise signal.        A particular case of the method is the one in which said at        least one characteristic of the noise signal is the frequency of        a high-level, narrowband component of the noise signal.

A further improvement of the method is the addition of a noisecancellation technique, which consists, as said above, in producing anoise signal equalized to the noise component contained in thecorresponding audio signal, and subtracting such equalized noise signalfrom the relevant audio signal. This cancellation technique is achievedby executing the following phases in place of the above filtering phasec

-   c1—noise component extraction phase, wherein at least one component    is extracted from the noise signal by means of a noise component    filter 209 implementing said bandpass filtering characteristic, and    a corresponding component is extracted from said filtered audio    signal by means of an audio component filter 208 implementing said    bandpass filtering characteristic;-   c2—equalization computation phase, wherein it is computed, by means    of a noise equalization processor 210, on the basis of said    extracted components and/or said noise signal and/or said filtered    audio signal, a delay and a noise equalization transfer function of    a filter equalizing at least the noise part contained in said at    least one component extracted from said noise signal to the noise    part contained in said at least one component extracted from said    filtered audio signal;-   c3—delay phase, wherein said filtered audio signal is delayed by    said delay computed in the previous phase by means of a delay filter    204;-   c4—noise equalization phase, wherein said noise signal acquired by    said noise acquisition means 102 is processed according to said    noise equalization transfer function by means of a noise    equalization filter 212;-   c5—cancellation phase, wherein in said delayed audio signal the    energy of said at least one component extracted from said noise    signal by means of a noise component filter 209 is reduced by means    of cancellation means 205 on the basis of at least said noise signal    processed by said noise equalization filter (212).    In a variant of the method, the stopband filter characteristic    and/or the passband filter characteristic is a multiband    characteristic, wherein the stopbands of a multiple stopbands filter    are regularly spaced by a first fixed spacing and/or the passbands    of a multiple passband filter are regularly spaced by a second fixed    spacing.

Moreover, the fractional bandwidth of the stopbands may be a first fixedamount and/or the fractional bandwidth of the passbands may be a secondfixed amount.

This solution makes it possible using motors and/or speed controllersaccording to state of the art, thereby not necessitating the use ofspeed sensors that would increase weight of the aircraft. In this way,it is possible to control the aircraft 100 using only the voice withoutusing a remote control according to the state of the art, so that it canadvantageously increase the safety and reduce theoscillations/vibrations of the aircraft.

In combination with the above-described features, the aircraft mayinclude electronic equipment (the so-called avionics), which maycomprise speed controllers capable of controlling engine speeds, flightcontrol means 213, a battery for supplying electrical energy to motorsand/or other electronic device or the like.

In a particularly advantageous variant, the above-described electronicdevices can be made partly or totally by utilizing the hardware alreadypresent in a mobile telecommunication device hosted on board of theaircraft. For example, the battery may be comprised in the mobiletelecommunication device; moreover, also the flight control means and/orprocessing means (e.g. a CPU) can be comprised in the mobiletelecommunication device, thereby exploiting the calculation power thatcan be provided by the microprocessors of the mobile telecommunicationdevice. In this case, the electrical connections between the motors ofthe aircraft and the electronics comprised in the mobiletelecommunication device may be made by means of a suitable plugconnector that connects to the output connector provided in the mobiletelecommunication device and wiring harness housed in chassis of theaircraft.

In this way, it is advantageously avoided the aircraft weight increasedue to the presence of an ad-hoc battery and/or an avionics providedoutside the mobile telecommunication device. The gyros andaccelerometers required to control the flight of the aircraft may alsobe those already comprised in said mobile telecommunication device, thusreducing the weight of the whole aircraft advantageously. In otherwords, the mobile telecommunication device comprises a battery suitablefor supplying energy to said aircraft, and/or said mobiletelecommunication device is configured to control the flight of saidaircraft, for example by generating appropriate motor control signals(directed to motors or speed controllers of the motors) on the basis ofthe accelerometers and/or gyroscopes outputs comprised in said mobiletelecommunication device.

This fact produces a reduction in weight that reduces the vibrationsgenerated by the motors, thereby reducing the vibration/oscillationamplitude to which said video acquisition means 21 of said mobiletelecommunication device are subjected during flight. In this way, thequality of the images produced by the video capture means 21 are lessdependent from the pilot's ability to fly the aircraft 100.

Alternatively or in combination with the above-described features, theaircraft 100 may comprise processing means configured for varying therotational speed of the aircraft motors in an opposite manner so as toincrease the signal-to-noise ratio of the audio signal acquired by theaudio acquisition means 101.

In particular, the processing means may be configured for varying therotational speeds of two or more motors in an opposite manner, i.e.increasing the rotational speed of one of the motors and reducing therotational speed of another motor, so that the air flow generated by theengine running at a lower speed will be lower and will produce anegligible amount of noise, while the air flow generated by the enginerunning at a higher speed (compared with the normal one) will producenoise having an advantageously higher frequency. This allows the inputfilter 203 to filter the signal more effectively, i.e. to obtain afiltered audio signal having a greater signal-to-noise ratio than thesolutions according to state of the art, as will be best described inthe following example.

In a quadcopter similar to aircraft 100, it is known that, in a hoveringflight condition, all motors rotate at similar speeds, for example equalto a rotation rate of 250 Hz, which corresponds to a rotational speed of15.000 rpm.

If the main frequency of this noise (along with its harmonics) made itimpossible to capture audio signals by the audio acquisition means 101(for example, because the upper harmonic of the motor rotation ratetriggers a resonance in the chassis of the aircraft, producing so muchnoise in the 1-2 kHz band that matches the audio signal band), it ispossible to increase the rotational speed of a pair of motors thatrotate in the same direction and, at the same time, decreasing therotational speed of the motor torque that rotates in the oppositedirection. In other words, the processing means may also be configuredto perform (during a motor speed adjustment phase of the methodaccording to the invention), before setting the filtering interval (i.e.prior to the filtering phase), the following steps:

-   -   increasing, by means of a first speed controller, the rotational        speed of a first motor;    -   decreasing, by means of a second speed controller, the        rotational speed of a second motor.

This avoids the triggering of frame vibration modes, by improving theacquisition of audio signals with the audio acquisition means 101, so asto enable the control of the aircraft 100 with the voice, even in thepresence of a frame having at least one resonant frequency that fallswith in the spectrum of audio frequencies, i.e. in the range of 300 Hzto 3.4 kHz.

This makes it possible to capture audio signals by the aircraft,reducing at least part of the noise from the vibration of the chassis.This also makes possible to use a speech recognition algorithm, enablingthe control of the aircraft without the use of dedicated remotecontrols.

Using a quadcopter similar to aircraft 100, the use of this solutionproduces an aircraft yawing in a particular direction, causing it torotate around its vertical axis (also referred to as the yaw axis). Thismovement can be advantageously used to indicate to the user of saidaircraft 100 that the audio acquisition means 101 can capture his/hervoice with a higher signal to noise ratio, i.e. can capture the audiosignal generated by the user's pressure waves speaking aloud.

It should be noted that this solution is also applicable to a coaxialbi-copter, a quadcopter, a hexa-copter, an eight-copter, or the other.Moreover, this solution can be advantageously applied to all multi-rotoraircraft having, for redundancy purposes, two motors coupled above andbelow the same housing location (e.g. a multirotor in an Y8configuration). Indeed, in this configuration (and also in thehexa-copter and octocopter configurations) it is also advantageouslypossible to avoid the aircraft yawing, because it is possible to balancethe reaction torque generated by using the (redundant) aircraft motors.

In another embodiment, the aircraft, which may comprise all the featuresdescribed above for all the previous embodiments, further comprisessource localization means (e.g. a microcontroller configured forcontrolling the direction of a directional microphone and/or a cameraconfigured for recognizing the user and generating positional data onthe basis of the user position in an acquired image) configured forperforming (during a spatial selection phase) the following steps:

-   -   identifying, on the basis of signals received from said audio        acquisition means 101 and/or other audio receiving means, a        position in a space of a source that produces pressure waves        generating said audio signal;    -   generating a pointing control signal on the basis of said        position.

More in details, the audio acquisition means 101 comprise a beam formingnetwork configured for selecting voice commands in said space on thebasis of said pointing control signal, wherein said beam forming networkreceives two or more inputs from distinct microphones, preferablymicrophones producing a pulse density modulation (PDM) output signal,and produces an output audio signal resulting from the selection of theportions of the audio signal produced by the pressure waves coming fromthe source position (identified during the spatial selection phase).

In this way, on board the aircraft it is possible to acquire audiosignals carrying voice commands and translate the voice commands intocontrol signals for the flight control means, so as to render thequality of the images captured by the video capture means independent ofthe user's piloting skills.

There are obviously many possible variants to the embodiments describedabove.

Some of the possible variants have been described above but it is clearto skilled person that, in the practical implementation, there are otherforms of realization, with different elements that can be replaced byother technically equivalent. The present invention is therefore notlimited to the illustrative examples described herein, but it is subjectto various modifications, improvements, replacement of parts andequivalent elements without departing from the basic inventive idea asspecified in the following claims.

The invention claimed is:
 1. A multi-rotor remote controlled aircraftfor capturing audio and/or video signals, comprising: at least a firstmotor that can be controlled by a motor control signal and coupled to afirst propeller, which is capable of generating a thrust for making saidaircraft flying, flight control means adapted to receive a commandsignal defining an attitude and/or a movement and/or an orientation ofthe aircraft, and to output at least one control signal for controllingsaid at least one motor on the basis of said command signal, audioacquisition means adapted to receive an audio signal carrying a voicecommand for said aircraft and at least one noise component, noisereduction means for reducing said at least one noise component,comprising: noise component selector configured for selecting at leastone filtering frequency of said at least one noise component of saidnoise signal on the basis of said at least one motor control signalreceived from said flight control means, wherein said at least onefiltering frequency represents the rotation rate of said first motor,filters setting means adapted to set filtering data defining at leastone stopband, wherein said at least one stopband comprises said at leastone filtering frequency or an harmonic thereof, input filter configuredfor filtering said audio signal on the basis of said filtering data, bygenerating a filtered audio signal, speech conversion means adapted toconvert said filtered audio signal, carrying a voice command, into acommand signal for said flight control means.
 2. The multi-rotor remotecontrolled aircraft according to claim 1, further comprising noiseacquisition means adapted to receive a noise signal coming from theenvironment surrounding said aircraft, and wherein said noise componentselector is also configured for setting the filtering frequency also onthe basis of at least one characteristic of said noise signal that saidnoise component selector detects in said noise signal.
 3. Themulti-rotor remote controlled aircraft according to claim 1, whereinsaid filters setting means are adapted to set second filtering data onthe basis of said filtering frequency, wherein said filtering frequencydefines at least one passband comprising the frequency of said at leastone selected noise component, and said noise reduction means furthercomprise: noise component filter and audio component filter configuredfor filtering, respectively, said noise signal and said filtered audioon the basis of said second filtering data, so as to extract,respectively, at least one component from said noise signal and acorresponding component from said filtered audio signal, noiseequalization processor configured for setting, on the basis of thesignal components extracted by said noise component filter and/or saidaudio component filter, equalization data defining a delay and a noiseequalization transfer function of a filter equalizing the noise partcontained in said at least one component extracted from said noisesignal to the noise part contained in said corresponding componentextracted from said audio signal filtered by said input filter, delayfilter configured for adding a delay to the filtered audio signal on thebasis of the equalization data, equalization filter configured forequalizing said at least one component extracted by the noise componentfilter from said noise signal on the basis of the equalization data,cancellation means configured for reducing, in the filtered audio signaldelayed by said delay filter, the energy of said at least one noisecomponent, on the basis of at least the output of said noiseequalization filter.
 4. The multi-rotor remote controlled aircraftaccording to claim 3, wherein said at least one stopband and/or said atleast one passband defines a plurality of bands, wherein the stopbandsof a multiple stopbands filter are regularly spaced by a first fixedspacing and/or the passbands of a multiple passband filter are regularlyspaced by a second fixed spacing.
 5. The multi-rotor remote controlledaircraft according to claim 4, wherein the fractional bandwidth of thestopbands is a first fixed amount and/or the fractional bandwidth of thepassbands is a second fixed amount.
 6. The multi-rotor remote controlledaircraft according to claim 3, wherein said equalization data are set bysaid noise equalization processor also on the basis of said noise signaland/or said audio signal filtered by said input filter.
 7. Themulti-rotor remote controlled aircraft according to claim 1, wherein thefiltered audio signal at the input of said speech conversion meansrepresents a voice command given by the user of said aircraft and saidspeech conversion means are configured to generate a control signal onthe basis of said audio signal received at its input by performing thesteps of executing a set of instructions implementing a speechrecognition algorithm, by converting said filtered audio signal into abit string, and generating the control signal based on said bit string.8. The multi-rotor remote controlled aircraft according to claim 1,further comprising source localization means configured for identifying,on the basis of signals received from said audio acquisition meansand/or other audio receiving means, a position in a space of a sourcethat produces pressure waves generating said audio signal, andgenerating a pointing control signal on the basis of said position,wherein said audio acquisition means comprise a beam forming networkconfigured for selecting voice commands in said space on the basis ofsaid pointing control signal.
 9. The multi-rotor remote controlledaircraft according to claim 1, comprising: a second motor that can becoupled to a second propeller capable of generating a thrust for makingsaid aircraft flying, a first speed controller adapted to control therotational speed of said first motor, a second speed controller adaptedto control the rotational speed of said second motor, and processingmeans in communication with said first and second speed controllers forregulating the rotational speed of said first and second motor, whereinsaid processing means are configured for increasing, by means of saidfirst speed controller, the rotational speed of the first motor, anddecreasing, by means of said second speed controller, the rotationalspeed of the second motor.
 10. An audio capture device, comprising:recording means capable of capturing an audio signal, transmission meansconfigured to transmit said audio signal to audio acquisition meanscomprised in a multi-rotor remote controlled aircraft according toclaim
 1. 11. A method for remote controlling a multi-rotor aircraft,comprising: a. an acquisition phase, wherein an audio signal carrying avoice command for said aircraft and at least one noise component arereceived by means of audio acquisition means, b. a frequency selectionphase, wherein, by means of a noise component selector, at least onefiltering frequency of at least one noise component of a noise signal isselected on the basis of the control signal of a first motor, whereinsaid at least one filtering frequency represents the rotational speed ofsaid first motor comprised in the aircraft, c. a filters setting phase,wherein at least one frequency stopband is set, by means of filterssetting means, on the basis of said at least one filtering frequency oran harmonic thereof, d. a filtering phase, wherein an audio signalacquired by audio acquisition means is filtered by an input filterfeaturing a stopband filter characteristic according to said at leastone frequency stopband, so as to reduce the energy of at least onecomponent of said audio signal having a frequency contained in said atleast one frequency stopband and to generate a filtered audio signal, e.a command generation phase, wherein a command signal is generated, bymeans of speech conversion means, on the basis of said filtered audiosignal, wherein said command signal defines an attitude and/or amovement and/or an orientation of the aircraft, f. a controltransmission phase, wherein said command signal is transmitted to flightcontrol means for controlling the flight of said aircraft.
 12. Themethod according to claim 11, wherein, during the frequency selectionphase, the noise component selector carry out the steps of: acquiring,by means of noise acquisition means, a noise signal representing theenvironmental noise surrounding said aircraft, selecting at least onefiltering frequency of at least one component of said noise signal alsoon the basis of at least one characteristic of said noise signal thatsaid noise component selector detects in said noise signal.
 13. Themethod according to claim 12, wherein said at least one characteristicof said noise signal is the frequency of a high-level, narrowbandcomponent of said noise signal.
 14. The method according to claim 11,wherein, during said filters setting phase, a second frequency band isset, by means of filters setting means, on the basis of said filteringfrequency and its harmonics, wherein said second frequency band definesa bandpass, and wherein said method further comprises: c1. a noisecomponent extraction phase, wherein at least one component is extractedfrom said noise signal by means of a noise component filter implementingsaid bandpass, and a corresponding component is extracted from saidfiltered audio signal by means of an audio component filter implementingsaid bandpass, c2. an equalization computation phase, wherein it iscomputed, by means of a noise equalization processor, on the basis ofsaid extracted components and/or said noise signal and/or said filteredaudio signal, equalization data defining a delay and a noiseequalization transfer function of a filter equalizing at least the noisepart contained in said at least one component extracted from said noisesignal to the noise part contained in said at least one componentextracted from said filtered audio signal, c3. a delay phase, whereinsaid filtered audio signal is delayed by said delay defined in theequalization data, by producing a delayed audio signal, c4. a noiseequalization phase, wherein said noise signal acquired by said noiseacquisition means is, by means of a noise equalization filter, equalizedthrough the noise equalization transfer function defined in saidequalization data, by producing an equalized noise signal, c5. acancellation phase, wherein, in said delayed audio signal, the energy ofsaid at least one component extracted from said noise signal is reduced,by means of cancellation means, on the basis of at least said equalizednoise signal.
 15. The method according to claim 14, wherein said atleast one stopband and/or said at least one passband defines a pluralityof bands, wherein the stopbands of a multiple stopbands filter areregularly spaced by a first fixed spacing and/or the passbands of amultiple passband filter are regularly spaced by a second fixed spacing.16. The method according to claim 15, wherein the fractional bandwidthof the stopbands is a first fixed amount and/or the fractional bandwidthof the passbands is a second fixed amount.
 17. The method according toclaim 11, comprising a spatial selection phase, wherein a position in aspace of a source that produces pressure waves generating said audiosignal is identified, by means of source localization means, on thebasis of signals received from said audio acquisition means and/or otheraudio receiving means, and wherein a pointing control signal isgenerated, by means of said source localization means, on the basis ofsaid position, and wherein, during the acquisition phase, said audiosignal is processed, by means of a beam forming network configured onthe basis of said pointing control signal, in order to select theportions of said audio signal produced by the pressure waves coming fromthe position identified during the spatial selection phase.
 18. Themethod according to claim 11, comprising a motor speed adjustment phase,wherein the rotational speed of the first motor is increased by means ofa first speed controller, and a rotational speed of a second motorcomprised in said aircraft is decreased by means of a second regulatorof speed, and wherein said motor speed adjustment phase is performedbefore the frequency selection phase.
 19. A computer program productwhich can be loaded into the memory of an electronic computer, and whichcomprises portions of software code for executing the phases of themethod according to claim 11.